This invention relates generally to methods and apparatus for nondestructively measuring porosity in composite structures.
The identification of internal flaws in large structures is critical to the safe use of these structures. For metal structures, the identification and characterization of melt-related inclusions and cracks are critical for lifing these parts. The inspection of large metal components led to the development of sophisticated technologies for detecting surface and volumetric defects. These technologies include x-ray, penetrant, and ultrasonic methods.
New product designs and manufacturing methods can create different types of defects than those generated during the manufacture of large metal structures. The design of new structures based on polymer matrix composites is one example of these new technologies. Composite structures have some unique flaws relating to the manufacturing process which do not exist with the manufacture of metallic structures. One of these flaw types is volumetric porosity. Undetected porosity can lead to early failures of critical components.
One known method for measuring porosity in composite structures is the use of acid digestion. With acid digestion, the weight percent of matrix material and fiber material are measured separately by using acid to dissolve one of the constituents. Using these data plus mass density information for the separate materials, the percent porosity can easily be determined. However, acid digestion methods are destructive because the composite must be dissolved in order to measure the volume of porosity. Acid digestion is valuable as a process control tool where either entire parts or sections of parts can be sacrificed to measure the capability of the manufacturing process. For most critical components for which safe operation is dependent on each component working properly, this destructive testing method cannot provide the needed level of porosity detection to assure safe operation. The actual structures must be measured.
Several researchers have studied the use of sound attenuation to estimate the porosity content in composites [1,2,3]. Nair, Hsu, and Rose [1] calculated the acoustic scattering caused by pores in a composite structure. They suggest the use of attenuation slope measurements for estimating porosity. They also provide experimental results that show an agreement between the experimental estimation of porosity, the theoretical calculation of the attenuation based on scattering theory, and actual porosity measurements collected using acid digestion methods,
Jeong and Hsu [2] continued work on the experimental analysis of attenuation slope measurements to estimate porosity. Jeong et al developed an immersion-based attenuation measurement technique that corrected for transducer diffraction and sound transmission losses. The researchers also identified that the attenuation slope measurement was sensitive to the shape or aspect ratio of the pores. This leads to three different coefficients for estimating porosity content from attenuation slope dependent on the construction technique for the composite structure.
Reed, Batzinger, Reed, and Jonsson [3] identified additional corrections needed for attenuation measurements made using focused immersion transducers. A correction for the surface roughness losses and a spatial filtering method to correct for frequency-dependent focusing effects were discussed. Experimental data showed agreement between the attenuation estimation of porosity and actual values determined by destructive sectioning of the sample.
All three groups demonstrated the applicability of using ultrasonic attenuation to estimate porosity in a laboratory setting. The data generally shows agreement between ultrasonic estimates for porosity measurement and actual values based on acid digestion or sectioning.
In general, known methods require precision scanning of two transducers collecting data at a plurality of frequencies. To collect the ultrasonic information needed to analyze porosity would require two or more scans of the part depending on the attenuation slope calculation method used, a serious limitation to manufacturing productivity. Additionally, two transducers are required for these measurements with their positioning axes. Since most immersion tanks designed for metal inspection have only one transducer manipulator, new immersion tanks with two fully controllable transducer manipulators would be required to implement these methods.
Another problem with the methods developed by these three groups is that the complexity of the calibration and measurements could make the inspection difficult for non laboratory-trained technicians. The diffraction correction techniques discussed by Jeong et al [2] require sophisticated mathematical skills including complex number mathematics. The focusing correction techniques used by Reed et al [3] require spatial convolutions of attenuation images to correct for focusing effects. These calculations would make the transfer of these techniques to a manufacturing environment very difficult.